Allowing for multiple stylesheets has never been so easy! Mozilla has a built in style changer (when you reference the alternate stylesheets correctly), but the other browsers don't. A List Apart put together a tutorial and a javascript styleswitcher a while back, and more recently, they added a PHP switcher.
Alternative Style: Working with Stylesheets – it explains how the script work, and then allows you to download it. It also gives examples of how to use it in your coding.
Build a PHP Switcher – gives a full tutorial on how to write up the code and the stylesheets to work with it. I originally saw a less polished tutorial of this on my css-d list: Backward-compliant style switcher (in PHP)
I have the javascript version working on the sidebar of my 1st skin on my blog: check out alt:colors on the sidebar. I look forward to playing with the PHP version because it will work even when Javascript is turned off on the browser.